The present invention relates to a method for conditioning polishing pads used in chemical-mechanical planarization of semiconductor wafers.
Chemical-mechanical polishing (xe2x80x9cCMPxe2x80x9d) processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is exposed to an abrasive medium under controlled chemical pressure, velocity, and temperature conditions. Conventional abrasive mediums include slurry solutions and polishing pads. The slurry solutions generally contain small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pads are generally planar pads made from a relatively porous material such as blown polyurethane, and the polishing pads may also contain abrasive particles to abrade the wafer. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer mechanically by the abrasive particles in the pad and/or slurry, and chemically by the chemicals in the slurry.
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. An under-pad 25 is typically attached to an upper surface 22 of the platen 20, and the polishing pad 40 is positioned on the under-pad 25. In most conventional CMP machines, a drive assembly 26 rotates the platen 20 as indicated by arrow A. In another existing CMP machine, the drive assembly 26 reciprocates the platen back and forth as indicated by arrow B. The motion of the platen 20 is imparted to the pad 40 through the under-pad 25 because the polishing pad 40 frictionally engages the under-pad 25.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows C and D, respectively.
In the operation of the CMP machine 10, the wafer 12 is positioned face-downward against the polishing pad 40 and at least one of the platen 20 or the wafer carrier 30 is moved relative to the other. As the face of the wafer 12 moves across the planarizing surface 42, the polishing pad 40 and the slurry 44 remove material from the wafer 12.
In the competitive semiconductor industry, it is desirable to maximize the throughput of the finished wafers and to minimize the number of defective or impaired devices on each wafer. The throughput of CMP processes is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is being planarized (the xe2x80x9cpolishing ratexe2x80x9d). Because the polishing period per wafer decreases with increasing polishing rates, it is desirable to maximize the polishing rate within controlled limits to increase the number of finished wafers that are produced in a given period of time.
CMP processes must also consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus the image of circuit patterns on the surface of the wafer. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the circuit pattern to better than a tolerance of approximately 0.1 xcexcm. Focusing the circuit patterns to such small tolerances, however, is very difficult when the distance between the lithography equipment and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, several devices may be defective on a wafer with a non-uniformly planar surface. Thus, CMP processes must create a highly uniform, planar surface.
One problem with CMP processing is that the throughput may drop, and the uniformity of the polished surface may be inadequate, because the condition of the polishing surface on the pad deteriorates while polishing a wafer. The deterioration of the polishing pad surface is caused by waste particles from the wafer pad, and slurry that accumulate on the polishing pad. The accumulations of waste particles effectively alter the condition of the polishing surface on the polishing pad causing the polishing rate to drift over time. The problem is particularly acute when planarizing doped silicon oxide layers because doping softens silicon oxide making it slightly viscous as it is planarized. As a result, accumulations of doped silicon oxide glaze the surface of the polishing pad with a glass-like material that substantially reduces the polishing rate over the glazed regions. Thus, it is often necessary to condition the pad by removing the waste accumulations from its polishing surface.
Polishing pads are typically conditioned with an abrasive disk that moves across the polishing pad and abrades the waste accumulations from the surface of the pad. One type of abrasive disk is a diamond-embedded plate mounted on a separate actuator that sweeps the plate across the pad. Some pad conditioners remove a portion of the upper layer of the deteriorated polishing surface in addition to the accumulations of waste matter to form a new, clean polishing surface. Other pad conditioners may use a liquid solution in addition to the abrasive disks to dissolve some of the waste matter as the abrasive disks abrade the polishing pad.
A more specific problem related to conditioning polishing pads is that conventional pad conditioning devices and processes significantly reduce the throughput of CMP processing. During conventional conditioning processes with abrasive disks, abrasive particles often detach from the abrasive disks and particles of pad material often detach from the pad. The detached abrasive particles or pad material may scratch the wafer if the wafer is not removed from the pad as it rotates during conditioning, or if the pad is not cleaned after it has been conditioned. More specifically, therefore, conventional conditioning processes with abrasive disks reduce the throughput of CMP processing because removing the wafer from the pad and cleaning the pad after conditioning requires down-time during which a wafer cannot be planarized.
In light of the problems associated with conventional polishing pad conditioning processes, it would be desirable to develop a process for conditioning polishing pads in which the wafer is not removed from the pad and the pad does not need to be cleaned after conditioning.
The inventive method conditions a polishing pad used in chemical-mechanical planarization of semiconductor wafers while the semiconductor wafer remains in situ on the polishing pad, and without necessitating cleaning after the pad is conditioned. In accordance with the method of the invention, waste matter on the polishing pad is dissolved with a conditioning solution selected to chemically dissolve the waste matter. The conditioning solution preferably coats the areas on the polishing pad upon which the waste matter tends to accumulate during planarization. After a desired amount of waste matter is dissolved into the conditioning solution to bring the pad into a desired condition without mechanically abrading the waste matter from the pad, the conditioning solution containing the dissolved waste matter is preferably removed from the pad.